In post-hemispherectomy children for seizure control, this proposal will examine motor and gait functions pre- and post- Body Weight Support Treadmill (BWST) training, and correlate changes from BWST training with functional MRI (fMRI) cortical activation from voluntary ankle movements. Our goals are to understand functional plasticity in the developing human brain by determining if age at injury/surgery, from specific pathologies, predicts baseline motor/gait functions and response to BWST training that can be used to design rehabilitation programs, using fMRI, that improve quality-of-life (QOL) for brain injured children. We hypothesize that injury/surgery from birth to 2 years maintains the developmental regulated ipsilateral corticospinal tract (ICS) that innervates ipsilateral muscles from primary motor/sensory (M1S1) cortex. By comparison, later injury/surgery, where the ICS tract has partially developmental regressed, will result in poorer functional recovery, especially for distal ankle/foot functions affecting walking. Based on Preliminary Data obtained from a Pilot Study we propose to determine if: 1) Age at injury/surgery related to pathology predicts motor and gait deficits pre- and post-hemispherectomy for proximal and distal muscles of the upper and lower limbs; 2) BWST training improves motor function and walking measures, if the changes are better in children with an early age at lesion/surgery, and if training induced walking and motor improvements are associated with better QOL measures; and 3) post-BWST gains of motor and walking function correlate with and increase in fMRI activation of M1S1 and cingulate cortex with ipsilateral and contralateral foot dorsiflexion consistent with cortical plasticity involving preservation of the ICS tract. The results of this research will have a conceptual impact regarding concepts of cortical plasticity in the developing human brain: and has practical application for neurorehabilitation in children post-hemispherectomy and for those with other forms of early developmental cerebral injury, such as trauma, hemorrhage, and tumors.